1. Field of the Invention
The present invention relates to a technology for monitoring and controlling oil return to a compressor, and more specifically, to a method and a system for monitoring and controlling quantity of a lubricating oil returned to an oil box of the compressor by controlling valve opening of an oil return valve at an oil return inlet of the oil box.
2. Description of the Related Art
Chillers are familiar refrigerating equipment in a central air conditioning system. Ice water produced by chillers efficiently lowers indoor temperature by heat exchange and via pipelines. In recent years, chillers are getting popular. A compressor is central to a chiller as far as the operation thereof is concerned. The compressor is a special air pump. The compressor in operation has to be continuously lubricated with lubricating oil in order to minimize friction. During a compression process of the compressor, a copious discharge of coolant (i.e., refrigerant) usually accompanies a loss of a trace of lubricating oil (known as “escaping oil”), which is inevitable. More badly, when discharged from the compressor, lubricating oil does not return to the oil box of the compressor in the end. As a result, the compressor tends to run out of lubricating oil. A shortage of lubricating oil causes friction to the detriment of bearings and then the compressor itself, thus damaging the chiller. Hence, the control of the lubricating oil return to the oil box is of vital importance to a central air conditioning system.
To return the otherwise discharged lubricating oil to the oil box, manufacturers usually put an oil return valve at the oil return inlet of the oil box, leave the oil return valve open to the full, that is, an opening of 100%, and drive the lubricating oil back to the oil box through the valve under a high pressure provided at a high-pressure outlet of the compressor at the cost of extra electrical power. However, the level of lubricating oil is not monitored and the number of the oil return is not controlled in the way as above. Even though the oil return is monitored, it still not ensures the lubricating oil return to oil box fully. Thereby the lubricating oil is decreasing with time. The operation of the compressor is hardly smooth because of low oil level. In addition, the system wastes a lot of electric energy because the oil return valve stays open. Hence, the system efficiency is going down.
In order to avoid low oil level and improve the system. U.S. Pat. No. 6,834,514 (FIGS. 6 and 7) and U.S. Pat. No. 6,993,920 (FIGS. 8 and 9) have disclosed an oil level sensor disposed in the oil box to detect the oil level and control the valve opening (0% or 100%, that is, by ON-OFF control) of an oil return valve provided at an oil return inlet of the oil box. The air conditioning system shuts down when the oil level is lower than an alert threshold. But the prior art still has drawbacks, for example, valve opening is switched between 0% and 100%, which is a rather great increment or decrement of valve opening. The compressor wastes a lot of electric energy and deteriorates the efficiency when the opening is 100%.
In conclusion, there is an urgent need for a method for overcoming the drawbacks of the prior art, so as to ensure a sufficiently high oil level, reduce high-pressure power loss, and render the system safe, reliable and efficient.